1. Field of the Invention
The present invention relates to an apparatus for wavelength dispersion and generation of a wavelength dispersion slope, and an apparatus for compensating for the wavelength dispersion accumulated in an optical fiber transmission network, and more specifically to an apparatus using a virtually imaged phased array for generating a wavelength dispersion and a wavelength dispersion slope.
2. Description of the Related Art
In the conventional fiber optical communications system for transmitting information through an optical system, a transmitter transmits a pulse to a receiver through an optical fiber. However, the wavelength dispersion of an optical fiber deteriorates the quality of a signal of a system.
To be more practical, as a result of wavelength dispersion, the transmission speed of a signal of an optical fiber depends on the wavelength of the signal. For example, if a pulse having a long wavelength (for example, the pulse of the wavelength indicating a red color pulse) is transmitted at a higher speed than a pulse having a short wavelength (for example, the pulse of the wavelength indicating a blue color pulse), it is normal dispersion. On the other hand, if a pulse having a short wavelength (for example, the pulse of the wavelength indicating a blue color pulse) is transmitted at a higher speed than a pulse having a long wavelength (for example, the pulse of the wavelength indicating a red color pulse), then it is abnormal dispersion.
Therefore, when a pulse contains red and blue pulses and is transmitted from a transmitter, the pulse is divided when it is transmitted through an optical fiber into a red pulse and a blue pulse which are received by a photodetector at different times. If a red pulse is transmitted at a higher speed than a blue pulse, it is normal dispersion.
If there are continuous wavelength components from blue to red as another example of pulse transmission, a pulse is extended in an optical fiber and distorted by wavelength dispersion because a red component and a blue component are transmitted at different speeds. Since all pulses contain finite wavelength extension, such wavelength dispersion frequently occurs in the fiber optical communications system.
Therefore, it is necessary for the fiber optical communications system to compensate for wavelength dispersion to obtain a higher transmission capacity.
To compensate for the wavelength dispersion, the fiber optical communications system requires an inverse dispersion device. Normally, an inverse dispersion device provides inverse dispersion for a pulse to nullify the dispersion generated by transmission through an optical fiber.
There are several devices which can be used as an inverse dispersion device. For example, a dispersion compensation fiber has a specific sectional index profile, thereby functioning as an inverse dispersion device and providing the inverse dispersion for nullifying the dispersion generated by the optical fiber. However, the dispersion compensation fiber is expensive in production cost, and has to be sufficiently long enough to successfully compensate for the wavelength dispersion. For example, when an optical fiber is 100 km long, the dispersion compensation fiber is approximately 20 through 30 km. Therefore, there is the problem of a large loss and size.
FIG. 1 shows the chirp fiber grating used as an inverse dispersion device to compensate for the chromatic dispersion.
As shown in FIG. 1, a ray is transmitted through an optical fiber, wavelength-dispersed, and then provided for an input port 48 of an optical circulator 50. The optical circulator 50 provides the ray for a chirp fiber grating 52. The chirp fiber grating 52 returns the ray to the optical circulator 50 such that different wavelength components can be reflected by the channel fiber grating at different distances, different wavelength components can travel different distances, and the wavelength dispersion can be compensated for. For example, the chirp fiber grating 52 can be designed such that a long wavelength component can be reflected at a long distance, and travel a longer distance than a short wavelength. Then, the optical circulator 50 provides the ray reflected to an output port 54 from the chirp fiber grating 52. Therefore, the chirp fiber grating 52 can add inverse dispersion to a pulse.
However, the chirp fiber grating 52 has very narrow band for a reflected pulse. Therefore, a sufficient wavelength band cannot be obtained to compensate for the ray containing a number of wavelengths such as a wavelength division-multiplexed light. A number of chirp fiber gratings can be cascaded for a wavelength division-multiplexed signal. However, the resultant system becomes costly. The chirp fiber grating obtained by incorporating a circulator is appropriate for a single-wavelength fiber optical communications system, etc.
FIGS. 2 and 3 shows the conventional diffraction grating used to generate wavelength dispersion.
As shown in FIG. 2, a diffraction grating 56 has a grating surface 58. Parallel rays 60 having different wavelengths are input into the grating surface 58. The rays are reflected by each stage of the grating surface 58, and interferes each other. As a result, rays 62, 64, and 66 having different wavelengths are output at different angles from the diffraction grating 56. The diffraction grating can be used in the spatial grating pair array described later to compensate for the wavelength dispersion.
FIG. 3A shows a spatial grating pair array used as an inverse dispersion device to compensate for wavelength dispersion.
As shown in FIG. 3A, a ray 67 is diffracted from a first diffraction grating 68, and becomes a ray 69 for a short wavelength and a ray 70 for a long wavelength. These ray 67 and ray 70 are diffracted by a second diffraction grating 71, and travel in the same direction. As shown in FIG. 3A, the wavelength components having different wavelengths travel different distances, thereby compensating for the wavelength dispersion. A long wavelength (such as the ray 70, etc.) travels a longer distance than a short wavelength. Therefore, the spatial grating pair array shown in FIG. 3A indicates abnormal dispersion.
FIG. 3B shows another spatial grating pair array used as an inverse dispersion device to compensate for chromatic dispersion.
As shown in FIG. 3B, lenses 72 and 74 are positioned between the first and second diffraction gratings 68 and 71. A long wavelength (such as the ray 70) travels a shorter distance than a short wavelength (such as the ray 69). Therefore, the spatial grating pair array shown in FIG. 3B indicates normal dispersion.
The spatial grating pair array as shown in FIGS. 3A and 3B are normally used to control the dispersion using a laser resonator. However, an actual spatial grating pair array cannot provide sufficient dispersion to compensate for a relatively large amount of chromatic dispersion generated by the fiber optical communications system. To be more practical, the angular dispersion generated by diffraction grating is normally very small, that is, approximately 0.05xc2x0/nm. Therefore, to compensate for the wavelength dispersion generated in the fiber optical communications system, the first and second diffraction grating 68 and 71 have to be largely apart. Accordingly, such a spatial grating pair array is not practical at all.
FIG. 4 shows the conventional technology of an inverse dispersion device using a VIPA.
In the above mentioned conventional technology, in the patent application numbers 10-534450 and 11-513133, the xe2x80x98Virtually Imaged Phased Arrayxe2x80x99 as shown in FIG. 4, that is, the device containing the portion referred to as VIPA 1, is suggested as an inverse dispersion device. The VIPA transmits from the VIPA the rays having different wavelengths spatially discriminated. This device includes an optical return device 2 for generating multiple reflection in the VIPA.
The above mentioned device can be realized by comprising a device including the VIPA 1 for receiving the input ray of a wavelength in the range of continuous wavelengths, and continuously generating a corresponding output ray. The output ray can be spatially discriminated from the output ray of a different wavelength in the range of the continuous wavelengths (for example, traveling in different directions). If the output ray can be discriminated at a forward angle, it proves that the device provides angular dispersion.
Furthermore, the above mentioned device is realized by comprising the VIPA 1 and the optical return device 2. The VIPA 1 contains a transmission area and a transparent element. By traveling through the transmission area, a ray can be input from and output to the VIPA 1. A transparent element 3 has the first and the second surfaces. The second surface is reflective, and passes a part of an input ray. The input ray passes through the transmission area. Then, it is received by the VIPA 1, and is reflected many times between the first and second surfaces of the transparent element. A plurality of the transmitted rays interfere each other, and an output ray 4 is generated. The input ray has a wavelength in the range of continuous wavelengths, and the output ray can be spatially discriminated from the ray of another wavelength in the range of the continuous wavelengths. The optical return device 2 can return the output ray in the completely opposite direction to the second surface, pass it through the second surface, and input it into the VIPA 1. The output ray is multiple-reflected in the VIPA 1, and output to an input path through the transmission area of the VIPA 1.
Furthermore, the above mentioned device can be realized by comprising a device including a VIPA for generating a plurality of output rays having the same wavelengths as the input ray and having different interference orders. The device also includes an optical return device for returning an output ray at one interference order to the VIPA, and not returning other output rays. Thus, only the ray corresponding to one interference order is returned to the VIPA.
In addition, the above mentioned device can be realized by including a VIPA, an optical return device, and a lens 5. The VIPA receives an input ray, and generates a corresponding output ray transmitted from the VIPA. The optical return device receives the output ray from the VIPA, and returns it to the VIPA. The lens is positioned such that: (a) the output ray can be returned from the optical return device to the VIPA by passing through the lens from the VIPA and being collected at the optical return device by the lens; (b) the output ray can be returned from the optical return device to the VIPA by being directed from the optical return device to the lens, and then to the VIPA by the lens; and (c) the output ray from the VIPA to the lens can travel parallel to and in the opposite direction of the output ray returned from the lens to the VIPA. Furthermore, the output ray from the VIPA to the lens does not overlap the output ray returned from the lens to the VIPA.
Furthermore, the above mentioned device can be realized by including the device having a VIPA, a mirror 6, and a lens. The VIPA receives an input ray, and generates an output ray traveling from a corresponding VIPA. The lens collects the output ray at the mirror, the mirror reflects the output ray, and the reflected ray is returned to the VIPA by the lens. The mirror is formed such that the device can perform constant wavelength dispersion.
As described above, the VIPA has the function of performing angular dispersion as diffraction grating, and can compensate for wavelength dispersion. Especially, it is characterized by large angular dispersion, and easily provides a practical inverse dispersion device. However, it requires specific conditions of a practical inverse dispersion device for use in a wavelength multiplexed transmission system.
FIG. 5 shows the wavelength dispersion of an optical fiber normally put for practical use.
The wavelength dispersion of an optical fiber normally put for practical use is not constant by a wavelength as shown in FIG. 5, but normally has a small positive slope (the longer the wavelength, the larger positive value the wavelength dispersion indicates). For example, in a common single mode fiber (SMF), the wavelength dispersion per 1 km is about +17 ps/nm while the wavelength dispersion slope indicates 0.06 ps/nm2. When a required wavelength band width is, for example, 35 nm, the wavelength dispersion changes approximately +2 ps/nm. Such a slope of wavelength dispersion can be referred to as a wavelength dispersion slope or a second order wavelength dispersion. The wavelength dispersion slope is not always positive (the longer the wavelength, the larger the wavelength dispersion), but a wavelength dispersion slope can be generated such that it can be negative in a wavelength extension longer than the zero dispersion wavelength in a dispersion shifted fiber with the zero dispersion wavelength shifted to the wavelength band of 1.5 xcexcm by changing the structure of the fiber.
Furthermore, the wavelength dispersion graph shown in FIG. 5 is not actually linear, and the slope of wavelength dispersion (wavelength dispersion slope) is not strictly constant, but the third order wavelength dispersion seldom causes problems at a transmission speed of about 40 Gb/s, and can be ignored.
Considering the actual wavelength dispersion of the transmission line of an optical fiber, the wavelength dispersion and a wavelength dispersion slope per unit length depend on the type of optical fiber as shown in FIG. 5, and the actual wavelength dispersion and wavelength dispersion slope depend on the length (transmission distance) of an optical fiber. If the wavelength dispersion of the actual optical fiber transmission line is compensated for by an inverse dispersion device, it is desired that a wavelength dispersion can be somewhat variable because the type and the transmission distance of an optical fiber largely depend on the period in which the optical fiber is installed and the situation of the site of the installed optical fiber.
In the wavelength multiplexed transmission, the compensation of the wavelength dispersion only is not sufficient, but a wavelength dispersion slope becomes a problem. Although the dispersion can be compensated for the wavelength of a signal channel, the wavelength dispersion cannot be completely compensated for with different wavelengths of signal channels if the wavelength dispersion of an inverse dispersion component is constant. Therefore, it is desired that the inverse dispersion device for the wavelength multiplexed transmission has a wavelength dispersion slope. Furthermore, since there are various transmission distances as described above, the wavelength dispersion slope changes in proportion to the length with the wavelength dispersion, it is also desired that the wavelength dispersion slope is somewhat variable. Furthermore, the value of a wavelength dispersion slope to be provided does not depend simply on the value of the wavelength dispersion because, if the type of optical fiber changes, not only the wavelength dispersion but also the wavelength dispersion slope changes as clearly shown in FIG. 5. That is, in the wavelength multiplexed transmission, it is the most desirable that the wavelength dispersion and the wavelength dispersion slope can be independently variable when the wavelength dispersion of an optical fiber transmission line is compensated for by an inverse dispersion device.
The method for having the wavelength dispersion and the wavelength dispersion slope independently vary is not clearly described by the above mentioned patent application numbers 10-534450 and 11-513133. Furthermore, it is not realized by the conventional inverse dispersion devices.
First, with a dispersion compensation fiber, an index profile including an inverse dispersion slope can be designed. However, changing its value requires various index profiles and lengths, and is not a practical operation. In addition, as described above, there is the problem that the method is costly, has a large loss, and large in size.
Furthermore, the chirp fiber grating also requires various index profiles and lengths although it can provide an inverse dispersion slope by optimizing the design of the chirp of the chirp fiber grating. Therefore, it is not a practical method, either. Although the wavelength dispersion and the wavelength dispersion slope are changed by changing the temperature, the value of the wavelength dispersion slope simply depends on the value of the wavelength dispersion. As a result, the wavelength dispersion and the wavelength dispersion slope cannot be independently varied. In addition, a sufficient wavelength band to compensate for the ray containing a large number of wavelengths such as a wavelength division-multiplexed ray cannot be obtained.
In the conventional diffraction grating, it is possible to independently vary the wavelength dispersion and the wavelength dispersion slope to a certain extent by appropriately arranging the diffraction grating. However, as described above, sufficiently large dispersion cannot be realized to compensate for a relatively large amount of chromatic dispersion generated in the fiber optical communications system in a practical size. Therefore, a practical method cannot be realized.
As described above, the present invention has been developed to solve the above mentioned problems, and aims at providing an apparatus for generating arbitrary wavelength dispersion and wavelength dispersion slope, and simultaneously performing dispersion compensation in multiple wavelength range of multiple channels on the wavelength dispersion and wavelength dispersion slope practically accumulated in an optical fiber.
The object of the present invention can be attained by providing an apparatus having the above mentioned VIPA including: a mirror formed such that substantially constant wavelength dispersion can be performed in a angular dispersion direction of the VIPA regardless of each wavelength of the output ray from the VIPA, and such that different wavelength dispersion can be performed in the direction substantially vertical to the angular dispersion direction of the VIPA; a unit for generating parallel gaps of optical paths by various wavelengths in the direction substantially vertical to the angular dispersion direction of the VIPA between the lens and the mirror; and a unit for varying the amount of the gaps of the optical paths.
According to the present invention, wavelength dispersion and a wavelength dispersion slope can be independently controlled, and can be provided for an optical signal. Therefore, an effective wavelength dispersion compensator can be provided when high-speed optical communications are realized using an optical fiber.